How do we repair this oops that hubby just made?

Since there is a plywood backer 1/8 " flat or diamond plate would do. I would get the damaged are expoxied back into position, sand it flat/flush. Cut the plate 2"-4" longer than the ladder is wide and 4"-6" vertical. Use Dicor for a seal and use 8 fairly large stainless wood screws to attach the plate, that way you can wipe the excess dicor off. Then do the ladder mount. If he backs into something again it will only bend the ladder or if harder that that a body shop will be needed.

LEN